Generally speaking, a voltage adapter, more particularly an AC/DC adapter, is used to receive a commercially available alternating-current (AC) power and convert the AC power into a direct-current (DC) power in order to enable a DC/DC converter to transform a high-level DC voltage into various low-level DC voltages, and thereby operating an electronic device. The electronic device which is adapted to cooperate with a DC power source can be, for example, a desktop computer.
The circuit configuration of a conventional AC/DC adapter is illustrated in FIG. 1. The AC/DC adapter as shown in FIG. 1 includes an input AC power source Vin, an EMI (electromagnetic interference) filter 11, a bridge rectifier 12, an input power detecting circuit 13, an energy storage device 14, a transistor switch Q1, a driver 15, and an output filter 16. The EMI filter 11 is constituted by an input power filtering capacitor X-cap, which is used to suppress the differential mode noise in the input AC voltage Vin. The input power filtering capacitor X-cap is connected with three serially-connected discharge resistors Rx in parallel, and the discharge resistors Rx are used to provide a discharge path for the input power filtering capacitor X-cap of the EMI filter 11 to release the energy stored therein. The bridge rectifier 12 comprises a plurality of diodes, and is used to perform a full-wave rectification to the input AC voltage Vin. The AC voltage generated by the bridge rectifier 12 is converted to a DC voltage by a DC/AC converter (not shown). The energy storage device 14 is used to store energy while the transistor switch Q1 is ON, and release the energy stored therein to the output filter 16 while the transistor switch Q1 is OFF. The output filter 16 comprises a diode D1 and a capacitor C1, and is used to perform a filtering and smoothing operation to the received DC voltage so as to provide an output DC voltage. The driver 15 is used to control on/off operations of the transistor switch Q1 according to a feedback amount derived from the output DC voltage, and thereby stabilize the output DC voltage. In addition, the AC/DC adapter further includes an input power detecting circuit 13 for sampling the input AC voltage and detecting the variation of the input AC voltage accordingly. The variation of the input AC voltage detected by the input power detecting circuit 13 is provided to the driver 15. Generally, the input power detecting circuit 13 comprises a voltage divider including a resistor R and a resistor r, whereas the resistance of the resistor r is far smaller than that of the resistor R.
Most of the portable electronic devices today are provided with power management facilities, and are able to adjust the allocation of the available power in response to user's demands and build the parameters used in standby mode operation. Thus, the portable electronic devices today are more power-saving than ever before and are able to work for a prolonged period. When the whole system is working under a standby mode, most of the internal circuit elements of the voltage adapter, including the transistor switch Q1 and the driver 15, are all shut down and stop operating. However, some passive circuit elements of the voltage adapter, for example, the discharge resistors Rx and the resistors R and r located within the input power detecting circuit 13, continue to receive the input AC voltage Vin and cause power consumption under standby mode. The senseless power consumption dissipated as a result of the passive circuit elements of the voltage adapter in standby mode operation will increase the overall power consumption of an electronic device.
Therefore, a major target to be pursued by the present invention is focused on the reduction of the power consumption resulted from the passive circuit elements, especially the resistors of the voltage adapter in standby mode operation.